The present disclosure relates to an image forming apparatus comprising a fixing unit configured to fix a toner image on a sheet.
An electrographic image forming apparatus, such as a copying machine or a printer, comprises a fixing device configured to fix a toner image on a sheet . In the fixing device, a heat fixing manner is generally applied to fix the toner image on the sheet by fusing a toner (a developer) with the heat. Such a fixing device with the heat fixing manner includes a first rotating member (for example, a roller or a belt) heated by a heating source and a second rotating member (for example, a roller) pressing the first rotating member to form a fixing nip between the first and second rotating members. Then, by passing the sheet through the fixing nip, the toner image is fixed on the sheet.
As the above-mentioned fixing device, a fixing unit configured to be attachable and detachable to an apparatus main body of the image forming apparatus may be applied. The fixing unit is designed in consideration of convenience of maintenance and part replacement.
On the other hand, in the above-mentioned fixing nip, the toner image may be fixed to various sheets with different thickness and dimensions. For instance, in a case where the toner image is fixed on a nonstandard sheet, such as an envelope or a thin paper, when a pressure (hereinafter, called as a “nip pressure”) of the fixing nip is the same as that of another case where the toner image is fixed on a standard sheet, there is fear that the sheet is crinkled. In recent years, particularly, the image forming apparatus is expected to correspond to speeding up and colorization, but it is difficult that one nip pressure satisfies conveying performance and fixing performance to various types of the sheets.
By contrast, a fixing unit varying a nip pressure in accordance with a sheet type is developed. For example, the fixing unit includes a pressing lever for holding a pressing roller, a compression spring for making the pressing roller pressed to a heating roller, and a cam for moving the pressing roller in a direction of increasing a shaft center distance between the pressing and heating rollers.
In this fixing unit, a gear is attached to a shaft connected with the cam, the gear is meshed with another gear of the apparatus main body, and moreover, a driving force from a driver installed to the apparatus main body is transmitted to the shaft via the above-mentioned gears so as to rotate the cam together with the shaft. As a nip pressure changing manner, except for a manner using the cam, other manners sliding a lever or operating a button are known.
In technique as mentioned above, because the fixing unit is provided with a nip pressure changing mechanism including the cam and shaft, the increasing size and complication of the fixing unit are incurred. In addition, on the structure locating the nip pressure changing mechanism in the fixing unit, because it is necessary to bear great load caused in changing the nip pressure by the fixing unit, there is fear that excessive load is applied to the fixing unit.
Further, the above-mentioned technique has a structure transmitting the driving force from the apparatus main body to the shaft via the apparatus main body side gear and fixing unit side gear. Therefore, there are fears that tooth jumping of each gear is caused and malfunction is caused by excessive shaft torque applied to each gear.